Project Summary/Abstract The mechanistic target of rapamycin complex 1 (mTORC1) is a serine/threonine kinase which coordinates cell growth and metabolism by balancing anabolic and catabolic processes in response to nutrients, growth factor signaling, and energy levels 1,2. Deregulated signaling in this pathway has been implicated in many human diseases including neurodegeneration, diabetes, and cancer3. As such, the mechanisms involved in mTORC1 activation and the biological processes under the control of mTORC1 are of significant interest. Lysosomes play a crucial role in the signal transduction of the nutrient sensing branch of the mTORC1 pathway9. While developing a novel tool for the rapid isolation of pure lysosomes, we made the surprising discovery that mTORC1 regulates the lysosomal concentration of a distinct set of non-polar, mostly essential amino acids. Using the same method, we noticed that loss of SLC38A9, a largely unstudied amino acid transporter that senses lysosomal arginine, results in the accumulation of the same set of non-polar, mostly essential amino acids. Despite this knowledge, the mechanism by which mTORC1 regulates the lysosomal abundance of essential amino acids through SLC38A9 is not known. Elucidation of this mechanism will give key insights into how mammalian cells respond to nutrient availability and may also provide novel therapeutic targets for the treatment of tumors. Preliminary evidence suggests the Rag-Ragulator complex, which interacts with SLC38A9 and conveys the availability of nutrients to mTORC1, is necessary for mTORC1 to regulate the efflux of essential amino acids from lysosomes. To characterize the molecular mechanism by which mTORC1 regulates SLC38A9 transport function, we propose the following aims: . 1. Determine the role of mTORC1 in controlling the interaction between SLC38A9 and Rag- Ragulator. 2. Understand the role of Rag-Ragulator phosphorylation on the regulation of the transport function of SLC38A9. 3. Identification of lysosomal proteins that regulate SLC38A9.